The performance of electronic circuits and their semiconductor devices (“chips”) is limited by temperature. Semiconductor device performance degrades when the internal temperature reaches or exceeds a particular limit. For example, in silicon integrated circuit devices, for each ten degree centigrade rise in junction temperature, the operating lifetime of the semiconductor device is decreased by a factor of at least two. Demands by OEMs for smaller package sizes and increased device densities has resulted in higher power densities, with the concomitant need for efficient heat dissipation becoming extremely important.
This industry need is compounded, in next generation, highly integrated semiconductor devices by the occurrence of “hot spots”, i.e., localized areas on the chip having relatively high thermal energy generation. These hot spots arise at locations on the chip where significant electrical activity occurs, e.g., processor, I/O control circuits, etc. The manner of cooling these devices has depended upon many parameters, including the space available for the cooling process, the temperatures to be encountered, the location(s) of hot spots, and the ability to distribute or “spread” the thermal energy over sufficient surface area to provide for efficient heat transfer. In the past, simply passing a fluid over the device or, over a finned heat sink that is attached to the device, was sufficient to maintain the semiconductor at safe operating temperatures. Different cooling fluids have been used, depending upon the application and the density of the electronic devices in a given circuit. Boiling liquids are often used, such as fluorinated hydrocarbon refrigerants, which are delivered to the semiconductor device in liquid form, and are then boiled to remove heat. These systems often have the highest heat removal rate for a limited area, but require a considerable amount of power to operate, i.e. to be pumped to and from the heat transfer site.
It is also well known in the art to employ heat pipes to cool semiconductor devices and packages. For example, in U.S. Pat. No. 4,697,205, issued to Eastman, a semiconductor circuit construction is provided in which the semiconductor junction is constructed as an integral part of a heat pipe to eliminate the package casing which tends to interfere with heat flow. The semiconductor chip material directly forms one wall of the casing of a heat pipe which is constructed as a hollow wafer-like configuration.
In U.S. Pat. No. 4,912,548, issued to Shanker et al., a housing is provided with a heat pipe that passes through the lid. The heat pipe terminates within the housing cavity at a hot end. A quantity of working fluid, such as fluorinated octane, is contained within the package cavity. The heat pipe communicates with cooling fins that produce a cold end. Heat from the semiconductor device inside the housing boils the working fluid. The fluid vapor passes along the heat pipe and is condensed at the cold end to be converted back to liquid.
In U.S. Pat. No. 5,097,387, issued to Griffith, a circuit chip package is disclosed which employs low eutectic or melting point solder as a thermally conductive medium between each circuit chip and the package cover. The package cover consists of a heat exchanger which includes a conventional heat pipe structure including a heat transfer fluid filled chamber, and a plurality of apertured pipes through which is passed another cooling fluid, such as air, to remove heat from the heat transfer fluid.
In U.S. Pat. No. 5,708,297, issued to Clayton, a multichip semiconductor module The is disclosed that is compatible with SIMM memory sockets. The multichip module includes a molded module frame and a composite semiconductor substrate subassembly received in a cavity in the frame. A cover plate and frame, alone or in combination, contain multiple compartments or channels through which gas or liquid coolant materials can be circulated to effectively distribute or remove heat generated from the semiconductor devices. In one embodiment, the cover plate includes a thin heat pipe.
In U.S. Pat. No. 5,780,928, issued to Rostoker et al., an electronic system is disclosed that provides thermal transfer from a semiconductor die in a semiconductor package by at least partially filling a cavity in the package with a thermally conductive fluid, immersing a heat collecting portion of a heat pipe assembly into the fluid, and sealing the cavity. In order that the thermally conductive fluid does not chemically attack the die or its electrical connections, the die and connections are completely covered with an encapsulating coating of an inorganic dielectric material by any of a variety of techniques. The heat pipe provides heat transfer from within the package to an external heat sink. In one embodiment, an absorptive wick is disposed within the package to transport condensed coolant to close proximity with the die.
In U.S. Pat. No. 5,880,524, issued to Xie, a package is provided for spreading the heat generated by a semiconductor device. The semiconductor device, such as a CPU, is mounted to a package substrate, a cover is attached to the package substrate creating a space therebetween for accommodating the semiconductor device. The package cover includes an external top surface and an external bottom surface and an inner cavity that comprises a heat pipe. The semiconductor device is thermally coupled to the bottom external surface of the cover.
These and other prior art devices use heat pipes to transfer thermal energy away from the semiconductor device and its package, but do not provide an efficient heat spreading mechanism for distributing the thermal energy generated by hot spots on the chip across the package itself. There is a need for a semiconductor packaging structure that efficiently and evenly spreads thermal energy generated by hot spots on a chip across a substantial portion of the package so that the thermal energy may be removed rapidly and effectively from the package.